As an exhaust gas purifying catalyst mounted on a vehicle and the like, a three-way catalyst has been known, which oxidizes or reduces harmful gas (hydrocarbon (HC), carbon monoxide (CO) and nitrogen oxide (NOx)) contained in exhaust gas, and converts the harmful gas into harmless water and gas. The three-way catalyst uses active alumina (Al2O3) having a large specific surface area as a support material, and supports noble metal particles of platinum (Pt), palladium (Pd), rhodium (Rh) and the like on a surface of the support material.
In recent years, emission control of the exhaust gas discharged from the vehicle and the like has been further tightened with the rise of ecological awareness, and accompanied with this, an improvement of the three-way catalyst has been advanced. For example, as an exhaust gas purifying catalyst excellent in purification performance for CO at low temperature, a catalyst which uses CeO2 as a support material and supports platinum on a surface of the support material has been developed. Such a catalyst can purify the exhaust gas in a relatively short time from an engine start.
Moreover, a catalyst has been developed, which uses CeO2—ZrO2 exerting an oxygen storage capacity (OSC) as a support material, thereby expanding a stoichiometric range and enhancing exhaust gas purification efficiency. The OSC is a capability of adsorbing oxygen in a lean atmosphere containing relatively much O2 and desorbing oxygen in a rich atmosphere containing much HC and CO. For example, an exhaust gas purifying catalyst has been disclosed, which supports palladium (Pd) on a complex oxide such as CeO2—ZrO2 to then form the catalyst, restricts sulfur poisoning and CO poisoning, and restricts sintering of the palladium, thereby enhancing a catalyst activity (refer to Japanese Patent Laid-Open Publication No. 2003-144923).
Furthermore, an exhaust gas purifying catalyst has been disclosed, which supports the noble metal and oxide particles of at least one metal selected from aluminum (Al), nickel (Ni) and iron (Fe) on the CeO2—ZrO2 complex oxide as the support material to then form the catalyst, and controls a movement of the noble metal by means of the oxide particles, thereby restricting growth of the particles of the noble metal (refer to Japanese Patent Laid-Open Publication No. 2003-126694).